Isobutylene is one of the most important monomers which polymerize by a cationic mechanism. Its polymers and particularly copolymers with dienes are the important industrial elastomers. The high molecular weight polymerization of isobutylene and its copolymerization with dienes to butyl rubber are commonly catalyzed by aluminium chloride or boron trifluoride at low temperatures, about -80.degree. to -100.degree. C. At higher temperatures the polymers and particularly the copolymers formed have lower molecular weights. Such copolymers with dienes are not suitable for further processing by vulcanization.
The disadvantage of the classic process for producing of butyl rubber is the high polymerization rate at the given low temperature. The high rate of polymerization prevents carrying out the polymerization at a high concentration of monomers or without solvent, with sufficient heat removal and protection from severe overheating of the polymerization mixture.
The aforesaid effect causes a decrease of the average molecular weight and broadening of the distribution of molecular weights in the product. For this reason, the process is carried out in dilute solutions with vigorous stirring positive cooling and with considerable volumes of auxiliary solvents, mostly chlorinated solvents, to assure solubility of the aluminium chloride catalyst. The process includes the recycling and purification of solvents. The inevitable low temperature necessary, which is energetically unsuitable, the purification of auxiliary solvents and the corrosion of plant apparatus by chlorinated solvents are the main negative characteristics of the known process for the production of butyl rubber.
Another limiting factor in the known low-temperature process for the production of butyl rubber is the choice of a suitable diene co-monomer. Many industrial processes are based on copolymerization of isobutylene with isoprene, in spite of the fact, that butadiene is a cheaper and more accessible diene than isoprene. Isoprene is used for its relatively more convenient copolymerization parameter to isobutene than butadiene. The copolymerization parameters for isoprene - isobutylene are essentially temperature independent, while the parameter of butadiene in copolymerization with isobutylene decreases with decreasing temperature. This prevents the preparation of butyl rubber by copolymerization of butadiene and isobutylene at the temperature condition of the known process, that is, at about -100.degree. C.